An experimentation with form, "Flip-Flop" was released as two discs (two episodes per disc) that could be listened to in either order. The story is cleverly constructed so that it makes sense either way.
It's Christmas Eve 3090. The Seventh Doctor and Mel turn up on the planet Punxatornee, where the human population exists as slaves to an alien slug race, the Slithergee. The local president, Bailey, has spent the past 30 years surrendering all human dignity to the Slithergee just to appease them. The Doctor and Mel don't plan to meddle, because they're right in the middle of a battle against some Quarks. They've only really popped over to Punxatornee in order to fetch some Applied Phlebotinum.
They're promptly captured by a man named Potter of the planetary police, who were already looking for them, believing them to be dangerous terrorists. A pair of humans (Reed and Stewart) watch the arrest. Stewart notes to Reed that Mel was right about having a time machine. So they bust the well-known terrorists out of jail, after a local Mad Doctor, Professor Capra, tortures the Doctor and Mel for a bit using an elaborate torture machine (that happens to use the Applied Phlebotinum that the Doctor was looking for). The Doctor and Mel are grateful but confused — first the police, and now Stewart and Reed, claim to know them. President Bailey is confused, too — how could the Doctor and Mel have escaped, considering that Stewart and Reed had been shot to death not an hour before?
Stewart explains that he and Reed are the last surviving members of La Résistance. Since the Doctor conveniently has a time machine, Stewart forces him (at gunpoint) to take them back 30 years into the past and prevent President Bailey from surrendering the planet to the Slithergee. They arrive just in time to find Bailey in a state of undress with her secretary, cheating on her secret boyfriend, Deputy Mitchell. Stewart kills both Bailey and her secretary, hoping to blame her death on the latter. Mitchell is left behind to become president and, mad with grief, declares war on the Slithergee.
The Doctor wearily takes everyone back to the future, where Stewart and Reed are horrified to find out that the planet has turned into a radioactive wasteland. Twenty years before, the planet was devastated by an attack from the Slithergee, who had been refused permission to settle there. As they rush into the city, hoping to find some proof that everything's going to be fine, the Doctor is forced to follow them when he realises that Stewart stole his Applied Phlebotinum (which the gang picked up over the course of the plot).
Back inside the TARDIS, Stewart forces the Doctor to take everyone back to just before he took everyone back the first time (even though the Doctor protests that that timeline doesn't exist anymore). The Doctor does so, and while Stewart and Reed rush off against the Doctor's advice, the Doctor and Mel are confronted by officer Potter, the same man who "recaptured" them when they arrived in this history — but from Potters perspective, this is the first time hes ever seen them. The Doctor therefore claims that he and Mel are Slithergee agents, and the bewildered Potter arrests them and takes them back to the central bureau.
Meanwhile, the Stewart and Reed that grew up in this timeline are lieutenants in the colony security forces. The Doctor figures that he has to be captured, since he was said to have escaped when they first arrived. Their quick escape (by decrypting the cells cipher lock) prompts the professor Capra of this timeline, who doesn't understand how anyone can know about his "machine", to issue a warrant for the arrest of the Doctor and Mel. Lieutenants Stewart and Reed are sent out to re-capture them. The Stewart and Reed from the Slithergee timeline quickly follow after them.
The Doctor realises that his self from this timeline is about to materialise his TARDIS in the exact same spot where the TARDIS is now, when he'll first arrive on Punxatornee. Since that would cause a paradox that could shred the Web of Time to pieces, he vworps out just in time to go deal with the Quarks, leaving the Doctor and Mel of this world to deal with the planet.
It's Christmas Eve 3090. The Seventh Doctor and Mel turn up on a radioactive wasteland that was once the planet Punxatornee. Twenty years before, the planet was devastated by an attack from the Slithergee, who had been refused permission to settle there. The Doctor and Mel don't plan to meddle, because they're right in the middle of a battle against some Quarks. They've only really popped over to Punxatornee in order to fetch some Applied Phlebotinum.
Two humans, lieutenants Stewart and Reed, recognise the Doctor and Mel as soon as they leave the TARDIS and immediately arrest them. They identify the Doctor and Mel by name and claim that theyve already confessed to being Slithergee agents and knowing of "the Professors machine". The Doctor is unable to decrypt his cells cipher lock; according to Stewart and Reed, hes escaped that way once already. However, they get rescued by other versions of Stewart and Reed (impersonating the lieutenants), who rather quickly get killed on the way back to the TARDIS.
Meanwhile, professor Capra has spent years forced to perfect a time machine for Mitchell, the planet's president. Mitchell plans to use the machine to go back in time and prevent the assassination of his secret girlfriend president Bailey by her secretary (and secret lover — Bailey was two-timing), 30 years ago. The Doctor realises that Capra's machine uses the Applied Phlebotinum he'll need to defeat the Quarks, but a confrontation in the lab results in both Mel and lieutenant Stewart being accidentally sent back 30 years into the past. Reed and the Doctor follow them in the TARDIS, but they're unable to prevent Stewart from murdering Bailey's secretary, and the (apparent) assassination attempt is prevented. The Doctor tries to force himself to shoot Bailey anyway, in order to preserve the Web of Time, but can't bring himself to go through with it. When Bailey learns from them what her planet would become if she were to refuse the Slithergee her hospitality, she solemnly vows to appease them as much as she can, in order to save humanity.
The Doctor wearily takes everyone back to the future, where Stewart and Reed are horrified to find out that the planet has turned into a slave colony for the now dominant Slithergee. As they rush into the city, hoping to find some proof that everything's going to be fine, the Doctor is forced to follow them when he realises that Stewart still has his Applied Phlebotinum (which he'd given to Stewart for safekeeping).
As the Doctor and Mel search for Stewart and Reed, they hear the security alert identifying them as enemy agents, and are then caught by this timeline's officer Potter. When he calls Lieutenant Stewart, however, the lieutenant claims that he and Reed are still on the way to collect their prisoners. Suspecting that Potter is in league with the enemy agents, Mitchell orders the lieutenants to return to Capras laboratory; he then contacts Capra himself, who seems somewhat distracted as Mitchell warns him to lock the door and keep an eye out for the Doctor and Mel.
Meanwhile, the Stewart from the radiation timeline orders the Doctor to take him and Reed back to the previous day so they can prevent themselves from going back in time. The Doctor and Mel try to point out that this is impossible, as the yesterday they came from no longer exists, but Stewart forces him. However, theyre forced to hide from Slithergee patrol 14 and Potter, the same man whom they saw killed when they arrived in this history. Stewart and Reed then arrive, demanding to know why the Doctor and Mel are breaking curfew — and when Mel greets Stewart and Reed by name, they are bewildered, as this is the first time theyve ever seen her. The Doctor realises that these are different versions of Stewart and Reed who grew up in this version of history, and this version of Stewart is very interested in Mels claim to have a time machine. When the Stewart and Reed from the radiation timeline are confronted by Potter, Stewart remembers — too late — that when they first arrived in this history, they met and killed Potter, who claimed to have killed them already. Potter shoots them both.
The Doctor realises that his self from this timeline is about to materialise his TARDIS in the exact same spot where the TARDIS is now, when he'll first arrive on Punxatornee. Since that would cause a paradox that could shred the Web of Time to pieces, he vworps out just in time to go deal with the Quarks, leaving the Doctor and Mel of this world to deal with the planet...
- Agony Beam: What Professor Capra's Mind Probe doubles as, and he is very happy to use it as such.
- All Take and No Give: The Slithergees in a nutshell. What was originally a moon being granted to them turned into most of the planet being covered by Slithergees, their culture, camps of human "sight guides", and even Slithergees on the human council, all while still stating they were being discriminated against.
- Alternate Timeline: Two that cross over each other.
- A Million Is Just A Statistic: One gets the feeling this kind of attitude is what makes the government so easily able to "economize" families, given that a deadly radioactive planet doesn't exactly grant one a lot of supplies to keep a population going...
- A Nazi by Any Other Name:
- The human regime on White Disc, though given the snowy and radioactive environment, it better ressembles Space Soviet Russia.
- The Slithergee on Black Disc.
- The Bad Guy Wins: No matter what action Bailey or Mitchell take, the Slithergee get the last laugh in the end.
- Bavarian Fire Drill: Stuart and Reed from the Black universe manage to double as their White universe counterparts a few times.
- Big Brother Is Watching You
- Bizarre Alien Senses: The Slithergees have very poor eyesight, but their sense of smell is potent enough they can smell something off about the TARDIS.
- Blatant Lies: Most things the Slithergee say just to get their way.
- Butterfly of Doom: No matter which universe, the actions of Stuart and Reed dooms the human population.
- Call-Back:The Doctor: "When I say run, run!"
- Christmas Episode: The story takes place on a pair of Christmas nights in both universes, although it isn't referred to as such in either universe.
- The Black universe calls the occasion "Slimetide Solstice", on behalf of the Slithergees, who take great offence at a small mention of Christmas.
- The White universe calls the occasion "Retribution Day", in remembrance of Bailey being assassinated by "Slithergee spies".
- Close-Enough Timeline: The Doctor tries to force one on the White disc, but can't bring himself to go through with it.
- Cold-Blooded Torture: The Black universe's Professor Capra is quite keen on doing this.
- Comically Missing the Point: Seven's still going through this phase, given he's travelling with Mel.The Doctor: "Changing time isn't like changing a light bulb!"Mel: "Why, what could happen!?"The Doctor: "Well you might burn your hand, or get electrocuted or fall off a stepladder."
- Crapsack World: Both worlds.
- Disadvantageous Disintegration
- Downer Ending: There's no happy ending for any of the humans on Punxatornee. The Black universe has them as slaves; the White universe has them dying out from radiation before the entire planet explodes.
- Earth-Shattering Kaboom: The time machine of the White universe had a rather bad problem of creating these when used owing to its power supply overloading. It wouldn't by so bad if the thing could be turned off!
- Fantastic Racism: The Slithergees keep asserting that they're being persecuted, regardless of the fact that the entire government keeps bending over backwards to give them everything they want.
- Faux Affably Evil: The Black universe Professor Capra offers a Christmas Cracker to the Doctor and Mel...before he hits them with an Agony Beam whilst humming Christmas carols.
- The Slithergee aren't much better.
- Gadgeteer Genius: Professor Capra in both universes, given one invents a Mind Probe and the other invents a time machine. The two are night and day, however.
- Genius Ditz: Mel really has her moments in this story.
- Have We Met Yet?: Twice over with Stuart and Reed, both in regards to Mel and the Doctor and the Potters of the opposite universe.
- Historical Hero Upgrade / Historical Villain Upgrade : Both In-Universe.
- Hourglass Plot: For the Slithergee, although it's a very wobbly hourglass which gets turned upside down a lot and sideways a few times.
- Insane Troll Logic: The Slithergee are full of this."Being a minority has got nothing to do with how many of us there are."
- In Spite of a Nail: Punxatornee's fate is pretty bleak, no matter what happens.
- Mind Screw: Quite possibly the most tangled single-story plotline(s) in Doctor Who history and, rather amazingly, it all makes sense in the end (or at the beginning, as it were). The whole story is built like an M. C. Escher painting.
- My God, What Have I Done?: Both sets of Stuart and Reed get this when they view what their actions have wrought.
- Mythology Gag: The anti-radiation gloves, which the Doctor claims were created by one of his previous incarnations, is a reference to a moment from "The Daleks". William Hartnell was supposed to say "anti-radiation drugs" but instead said "anti-radiation gloves".
- Noodle Incident: From the Doctor's point of view, this is a small side-step in the middle of a much larger (and never specified) adventure.
- Pinball Protagonist: Mel and the Doctor bounce between two universes...both sets of them!
- Puppet King: President Bailey is effectively one in the Black Disc, given her efforts to appease the Slithergees, before she takes her life after realizing Mitchell going behind her back. Mitchell briefly becomes one, before his life is taken by the Slithergees.
- Rewarded as a Traitor Deserves: The Black universe's Stuart and Reed execute Bailey and her assistant for giving into the Slithergees. The White universe's simply kill her assistant, under the impression he was going to assassinate her.
- Running Gag: "Bigger on the Inside"
- Set Right What Once Went Wrong: What Stuart and Reed attempt to do in both universes. Neither are happy with what they get.
- The name "Punxatornee" is a reference to Groundhog Day. Professor Capra is named for Frank Capra, director of the classic Christmas movie It's a Wonderful Life. The other characters are also named for characters and actors from It's a Wonderful Life.
- The bells after the announcement on Black Disc chime in "We're all having a wonderful Christmas time.", which Mel notes. She later namedrops Bing Crosby and Edgar Allan Poe.
- Stable Timeloop: Although it's more like a Moebius Strip. You have to go around the loop twice to get back to your starting point.
- Stun Guns: The White universe's Stuart and Reed are armed with these, although they have Disintergrator Ray functions. The Black universe goes for much simpler and deadlier ray guns.
- Temporal Mutability: Both the Doctor and Mel don't seem bothered they've settled into another timeline, either time. Stuart and Reed...don't get a chance to settle in, either time.
- Temporal Paradox: Stuart and Reed only go back in time to fix the actions that another Stuart and Reed caused, made possible because their counterparts encounter the Doctor and Mel as they land on Puxatournee.
- Tempting Fate: Stuart and Reed really should have paid better heed to what Mel and the Doctor were saying.
- Reed's grandfather was cut down in both universes for thinking it would have been better if the government went with the opposite decision.
- Terminator Twosome: The Black and White teams of Stuart and Reed, from each other's point of view.
- Time Machine: Professor Capra's invention on the White Disc.
- Trigger Happy: Both Stewarts try to solve everything by shooting at it or threatening to. Black Stewart is put in a pickle by this when his gun runs out of charges because of him overusing it, and he's rather unceremoniously shot dead. White Stewart is barely more restrained and polite, possibly from his military upbringing. Black Reed is almost as bad as Stuart, but White Reed is much more composed and collected.
- Theme Naming: Mostly from It's a Wonderful Life.
- Trapped in the Past: Mel.
- We Come in Peace Shoot to Kill: The Slithergee come to Puxtawnee and claim they're refugees. With a Battle Cruiser. It's part of what made Bailey hesitate in giving them one of the moons to colonize.
- Whole Plot Reference: To Terminator, Groundhog Day and It's a Wonderful Life. All three films are cleverly alluded to.
- Why Don't You Just Shoot Him?: Subverted.
- Wounded Gazelle Gambit: What the Slithergees revert to when separated from their human sight guides, or when being confronted at all about anything on a fair footing.
- You Have Outlived Your Usefulness: What the Slithergees decide for Mitchell soon after Bailey dies.
- You Mean X Mas: On Black Disc the Slithergee wish to supplant Christmas with "Slimetide Solstice". On White Disc it's "Retribution Day".